1. Field of the Invention
The invention relates to imaging sensors and more particularly to a color system for imaging sensors.
2. Description of Related Art
Digital color imaging devices are routinely built by overlaying color filter arrays (CFA) on a charge coupled device (CCD) or complimentary metal oxide semiconductor (CMOS) imagers. In general, a CFA is made of three or more color channels that are each selective to visible light and therefore limit the light that is incident on an imaging sensor. The color filter channels selectively allow light corresponding to a predetermined range of the visible spectrum to pass through a channel to the imaging sensor. Thus, a CFA allows light having the color content of the imaged scene to be estimated by appropriate operation of the device on the responses of the camera colored channels. Commonly used color schemes include red-green-blue (RGB), cyan-magenta-yellow (CMY), and cyan-magenta-yellow-green (CMYG). Color schemes employing a "white" or uncolored pixel, have also been reported. These include a cyan-white-yellow-green (CWYG) color system reported by Hitachi Ltd., Mobara Works of Mobara, Japan and a cyan-white-green (CWG) system reported by RCA Laboratories of Zurich, Switzerland.
The considerations for any color system generally include the color fidelity, the signal-to-noise ratio of the imaging system, and the manufacturability of the sensor. Color fidelity deals with the capability of the color system to accurately measure or reproduce the color of the imaged scene, i.e. the ability to render a colorimetrically accurate image. The signal-to-noise ratio of the imaging system, e.g., camera system deals with the number of signals delivered by the sensor to the system that are attributable to photons and thus the imaged scene. Finally, the manufacturability of the sensor is concerned with the various processing steps, costs, and yield in the manufacturing of a particular sensor for an imaging system.
Color fidelity is a consequence of the CFA characteristics. The goal of any color system is to match or predict the human eye response. A color system is successful if it can match the color sensors of the eye or if the system is made up of colors that have enough difference between them over the visible spectrum that, through a mathematical manipulation, the system can effectively predict the human eye response. It is to be appreciated that sophisticated mathematics can be used to adapt any color combination to the human eye response. However, sophisticated mathematics are not ordinarily used because of the degree of mathematics required at each of the thousands of pixels of a given image, i.e., the time factor, and the concern of increasing or multiplying the amount of noise present in any sensor. Thus, for simplicity and practicality, color systems selected based on their ability to match the human eye response through a mathematical manipulation are generally limited to such color systems that allow such manipulation to occur via a 3.times.3 matrix transformation of the numerical data as opposed to larger matrix transformations.
As mentioned above, no image is free of noise. In a digital sensor, photons strike the sensor and the sensor converts the photons, generally through the use of a photodiode or similar device, to electrons. However, as yet, electrical sensors are not perfect and some electrons pass through the diodes as leakage or dark current that are not representative of the photons of the imaged scene. Thus, when the final signal of the imaged scene is collected, a leakage or dark current is present and it is difficult to distinguish an electron originating from a photon from an electron that is attributable to leakage or dark current. The leakage or dark current is characterized as a source of noise. There are other sources of noise, including a source of noise resulting from a quantization error of reducing the information content of a signal to its digital value.
The above sources of noise allow an evaluation of the signal-to-noise ratio of a color system. An RGB system, for example, divides the range of the visible spectrum that a human eye can see (approximately 400-700 nanometers) into three colors based on their frequency and wave lengths, RGB. Each Red, Green, or Blue filter passes about one-third of the available light through its color channel. Therefore, each color channel ignores two-thirds of the available photons for an imaged scene. Thus, two-thirds of the light that is incident on the filter goes to waste and is never used to create a signal in the camera. The amount of noise attributable to the sensor can be reduced if each color channel is exposed to more of the available photons.
A CMY color scheme or color system is known as a complimentary set of colors. By complimentary, the individual colors Cyan, Magenta and Yellow, provide a broader spectrum than either Red, Green, or Blue. Cyan is a combination of blue and green. Magenta is a combination of blue and red. Yellow is a combination of green and red. In effect, each Cyan, Magenta, or Yellow passes two-thirds of the available light of an imaged scene. Only one-third of the photons are ignored on a given color channel. This would appear to decrease the noise by a factor of two over the RGB system, since twice as much light is seen by the sensors as an RGB system. However, the reduction in noise benefit must be reduced somewhat by the mathematical manipulation of the color channels to predict the human eye response. In other words, the complimentary scheme CMY requires more mathematical manipulation than the non-complimentary RGB color scheme and, because there is still some noise present in the captured image, that noise is increased through the mathematical manipulation.
An RGB color scheme is often selected for ease of signal processing and generally good color fidelity. CMY and CMYG systems are often employed where it is desirable to improve the light sensitivity of the camera system by maximizing the integrated transmittance of the color filters over the visible spectrum. Because additional signal processing is necessary to recover an RGB color signal suitable, for example, to display on a computer monitor, from CMY or CMYG systems, the color fidelity is sometimes compromised in favor of an improved signal-to-noise ratio.